test.rb

gem 'active_model_serializers', github: 'rails-api/active_model_serializers', branch: '0-10-stable'

class Item
  include Mongoid::Document
  include Mongoid::Timestamps
end

def index
  render json: Item.all
end

-----

Failure/Error: get 'index'
NoMethodError:
undefined method `new' for nil:NilClass

Since there is no serializer for the class Item this line https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer/array_serializer.rb#L17 will be nil